Slow CO Research Articles

Large Eddy Simulation of a Pressurized, Partially Premixed Swirling Flame With Finite-Rate Chemistry

Finite-rate chemical effects at gas turbine conditions lead to incomplete combustion and well-known emissions issues. Although a thin flame front is preserved on an average, the instantaneous flame location can vary in thickness and location due to heat losses or imperfect mixing. Postflame phenomena (slow CO oxidation or thermal NO production) can be expected to be significantly influenced by turbulent eddy structures. Since typical gas turbine combustor calculations require insight into flame stabilization as well as pollutant formation, combustion models are required to be sensitive to the instantaneous and local flow conditions. Unfortunately, few models that adequately describe turbulence–chemistry interactions are tractable in the industrial context. A widely used model capable of employing finite-rate chemistry is the eddy dissipation concept (EDC) model of Magnussen. Its application in large eddy simulations (LES) is problematic mainly due to a strong sensitivity to the model constants, which were based on an isotropic cascade analysis in the Reynolds-averaged Navier–Stokes (RANS) context. The objectives of this paper are: (i) to formulate the EDC cascade idea in the context of LES; and (ii) to validate the model using experimental data consisting of velocity (particle image velocimetry (PIV) measurements) and major species (1D Raman measurements), at four axial locations in the near-burner region of a Siemens SGT-100 industrial gas turbine combustor.

Storage of CO2 into Porous Coordination Polymer Controlled by Molecular Rotor Dynamics.

Design to store gas molecules, such as CO2 , H2 , and CH4 , under low pressure is one of the most important challenges in chemistry and materials science. Herein, we describe the storage of CO2 in the cavities of a porous coordination polymer (PCP) using molecular rotor dynamics. Owing to the narrow pore windows of PCP, CO2 was not adsorbed at 195 K. As the temperature increased, the rotors exhibited rotational modes; such rotations dynamically expanded the size of the windows, leading to CO2 adsorption. The rotational frequencies of the rotors (k≈10-6 s) and correlation times of adsorbed CO2 (τ≈10-8 s) were elucidated via solid-state NMR studies, which suggest that the slow rotation of the rotors sterically restricts CO2 diffusion in the pores. This restriction results in an unusually slow CO2 mobility close to solid state (τ≥10-8 s). Once adsorbed at room temperature, CO2 is robustly stored in the PCP under vacuum at 195-233 K because of the steric hindrance of the rotors. We also demonstrate that this mechanism can be applied to the storage of CH4 .

Storage of CO2 into Porous Coordination Polymer Controlled by Molecular Rotor Dynamics

AbstractDesign to store gas molecules, such as CO2, H2, and CH4, under low pressure is one of the most important challenges in chemistry and materials science. Herein, we describe the storage of CO2 in the cavities of a porous coordination polymer (PCP) using molecular rotor dynamics. Owing to the narrow pore windows of PCP, CO2 was not adsorbed at 195 K. As the temperature increased, the rotors exhibited rotational modes; such rotations dynamically expanded the size of the windows, leading to CO2 adsorption. The rotational frequencies of the rotors (k≈10−6 s) and correlation times of adsorbed CO2 (τ≈10−8 s) were elucidated via solid‐state NMR studies, which suggest that the slow rotation of the rotors sterically restricts CO2 diffusion in the pores. This restriction results in an unusually slow CO2 mobility close to solid state (τ≥10−8 s). Once adsorbed at room temperature, CO2 is robustly stored in the PCP under vacuum at 195–233 K because of the steric hindrance of the rotors. We also demonstrate that this mechanism can be applied to the storage of CH4.

The Effect of Surface Roughness on Diffusion and Chemical Reaction Controlled Limiting Currents on a Rotating Cylinder Electrode in Deaerated Solutions with and Without CO2

The influence of surface roughness on mass transfer on a rotating cylinder electrode apparatus is investigated experimentally for a roughness pattern consisting of grooves parallel to the direction of fluid flow. Mass transfer from four different samples, with roughness values of 0.5 μm, 6 μm, 20 μm, and 34 μm, is measured using the limiting current technique for a range of rotational speeds in NaCl solutions saturated with N2 at pH = 3 and 4. Comparison with available correlations for the Sherwood number in literature (which are independent of surface roughness and are either for specific or arbitrary roughness patterns) shows that H+ mass transfer only correlates well for particular levels of roughness and that their accuracy can be increased if a correlation is utilized which is a function of surface roughening. A new correlation for Sherwood number as a function of the Reynolds number, Schmidt number, and surface roughness is proposed which agrees well with the mass transfer observed from all of the rough surface cases considered for this particular roughness pattern. Complementary experiments in CO2 environments were used to assess the combined limiting current associated with H+ and H2CO3 reduction (with the latter occurring via the buffering effect and being associated with the slow CO2 hydration step). Although the increase in sample roughness clearly leads to an increase in the rate of H+ mass transfer, in the CO2 environments considered, surface roughness is found to have no significant influence on the limiting current contribution from H2CO3, which can therefore be determined from Vetter’s equation across this range of operating conditions.

ROS-mediated carbon monoxide and drug release from drug-conjugated carboxyboranes.

Elevated levels of reactive oxygen species (ROS) are associated with several detrimental diseases. Therefore, using this condition for selective treatments could be a great advantage in fighting these disorders. Here, we present amine carboxyboranes as a class of molecules for therapeutic CO gas generation and for delivering drug molecules in the presence of ROS. Our findings indicate amine carboxyboranes to be very slow CO releasers at physiological pH and temperature without any inducers but generate CO faster when reacted with a non-radical ROS (H2O2). Furthermore, a radical ROS (˙OH) dramatically expedites the production of CO from amine carboxyboranes. A decomposition profile shows the release of the amine group during the decarbonylation process from drug-conjugated carboxyboranes. This suggests that the carboxyborane moiety can be a useful tool for the delivery of amine-containing drugs to disease sites which display abundant ROS levels.

Hexamethylenetetramine carboxyborane: synthesis, structural characterization and CO releasing properties.

Carbon monoxide, although widely known as a toxic gas, has received great attention in the past few decades due to its promising role as a medical gas. Several classes of carbon monoxide releasing molecules (CORMs) have been synthesised with many of them having pharmacological activities under physiological conditions. Herein, we report the synthesis and structural characterization of the first example of amine carboxyborane that releases CO under physiological conditions without the aid of inducers. A representative compound hexamethylenetetramine carboxyborane (HMTA-CB) described here has a half-life of 2.7 days and gradually releases CO with the rate constant of 3.0 × 10-6 s-1. Its ability to promote cell growth shows the beneficial effect of slow CO release to supplement CO in small amounts over time.

Analysis of the pH-dependent thermodynamic stability, local motions, and microsecond folding kinetics of carbonmonoxycytochrome c

This paper analyzes the effect of pH on thermodynamic stability, low-frequency local motions and microsecond folding kinetics of carbonmonoxycytochrome c (Cyt-CO) all across the alkaline pH-unfolding transition of protein. Thermodynamic analysis of urea-induced unfolding transitions of Cyt-CO measured between pH 6 and pH 11.9 reveals that Cyt-CO is maximally stable at pH∼9.5. Dilution of unfolded Cyt-CO into refolding medium forms a native-like compact state (NCO-state), where Fe2+−CO interaction persists. Kinetic and thermodynamic parameters measured for slow thermally-driven CO dissociation (NCO→N+CO) and association (N+CO→NCO) reactions between pH 6.5 and pH 13 reveal that the thermal-motions of M80-containing Ω-loop are decreased in subdenaturing limit of alkaline pH. Laser photolysis of Fe2+-CO bond in NCO-state triggers the microsecond folding (NCO→N). The microsecond kinetics measured all across the alkaline pH-unfolding transition of Cyt-CO produce rate rollover in the refolding limb of chevron plot, which suggests a glass transition of NCO en route to N. Between pH 7 and pH 11.9, the natural logarithm of the microsecond folding rate varies by < 1.5 units while the natural logarithm of apparent equilibrium constant varies by 11.8 units. This finding indicates that the pH-dependent ionic-interactions greatly affect the global stability of protein but have very small effect on folding kinetics.

Electrocatalytic CO2 Reduction on Nanosized Ag Dendrites Electrodeposited on Indium Doped SnO2 Electrodes Immersed in Aprotic Medium

Electrochemical reduction of CO2 by means of metallic nanostructures (i.e. Ag, Au, Cu), sp2 carbon derivatives (i.e. graphite oxide, graphene) and their combinations is considered as a promising pathway for aiding to decrease the effects of the global warming. However, the electrochemical reduction of CO2 contained in aqueous medium requires large overpotentials with a significant competition with the H2 evolution reaction, thus resulting in low faradaic efficiencies. On the other hand, reactivity of metallic nanostructures is strongly dependent from the size and the aspect ratio in nanoparticles and nanorods, respectively, as well as from the crystalline faces exposed to the electrolytic medium. In order to overcome this trouble, we studied the electrocatalytic CO2 electroreduction in aprotic medium by employing nanosized Ag dendrites, which were previously electrodeposited on In-doped SnO2 (ITO) electrodes by means of a double-pulse potential (DPP) technique (deposition conditions were defined by means of cyclic voltammetry (CV) technique). These experiments were carried out in a three-electrode cell containing aqueous 0.1M KNO3+1mM AgNO3. In this sense, the presence of dendritic nanostructures on the silver-modified ITO electrodes (ITO/Ag) was confirmed by scanning electron microscopy. CV results showed that the electrical charge associated to the Ag dissolution is clearly dependent from the growth time (duration ~ms) and independent from the nucleation time (duration ~sec). This result demonstrated that the Ag deposition is kinetically-controlled by one-electron transfer. Furthermore, CV results demonstrated that the thermodynamic potential for the Ag reduction E°Ag+|Ag was observed at -0.32 vs. Ag|AgCl (3M NaCl) during the first scan but, it becomes less cathodic at E°Ag+|Ag= -0.08 vs. Ag|AgCl (3M NaCl) during the successive scans; thus confirming that the dendritic shape of the Ag deposits is promoted immediately after some seeds of metallic atoms are confined on the ITO surface. Complementarily, electrochemical CO2 reduction in a three-electrode cell containing propylene carbonate + 0.1M Bu4NPF6 was confirmed on the as-prepared ITO/Ag electrodes by means of linear voltammetry (LV). In this way, two reduction peaks were localized at -0.3 and -0.5V vs. Ag|AgCl (0.01M AgNO3 + 0.1M Bu4NCl in MeCN), which can be associated to the semireactions 1 and 3, respectively. Electrochemical contribution from the ITO substrate was absent during the CO2 reduction reaction. (1) CO2 + e- → CO2 · - (slow)(2) CO2 + CO2 · - → (CO2)2 · - (3) (CO2)2 · - + e- → -OOC-COO· -

Carbon monoxide bioconversion to butanol-ethanol by Clostridium carboxidivorans: kinetics and toxicity of alcohols

Butanol production from carbon monoxide-rich waste gases or syngas is an attractive novel alternative to the conventional acetone-butanol-ethanol (ABE) fermentation. Solvent toxicity is a key factor reported in ABE fermentation with carbohydrates as substrates. However, in the gas-fermentation process, kinetic aspects and the inhibition effect of solvents have not thoroughly been studied. Therefore, different batch bottle experiments were carried out with the bacterial species Clostridium carboxidivorans using CO as carbon source for butanol-ethanol fermentation. A maximum specific growth rate of 0.086±0.004h(-1) and a biomass yield of 0.011 gbiomass/gCO were found, which is significantly lower than in other clostridia grown on sugars. Besides, three assays were carried out to check the inhibitory effect of butanol, ethanol, and their mixtures. Butanol had a higher inhibitory effect on the cells than ethanol and showed a lower IC50, reduced growth rate, and slower CO consumption with increasing alcohol concentrations. A concentration of 14-14.50g/L butanol caused 50% growth inhibition in C. carboxidivorans, and 20g/L butanol resulted in complete inhibition, with a growth rate of 0h(-1). Conversely, 35g/L ethanol decreased by 50% the final biomass concentration respect to the control and yielded the lowest growth rate of 0.024h(-1). The inhibitory effect of mixtures of both alcohols was also checked adding similar, near identical, concentrations of each one. Growth decreased by 50% in the presence of a total concentration of alcohols of 16.22g/L, consisting of similar amounts of each alcohol. Occasional differences in initially added concentrations of alcohols were minimal. The lowest growth rate (0.014h(-1)) was observed at the highest concentration assayed (25g/L).

PCC pilot plant studies with aqueous potassium glycinate

The present research work discusses the suitability of aqueous potassium glycinate (KGly) in a PCC-pilot plant. Reference is made to measurement results with aqueous sodium glycinate (NaGly). The use of real power plant flue gas and the well-conceived dimensions of the test facility enable industry-related conditions for full-scale applications.Although 40wt% KGly has faster kinetics in unloaded state, no enhancements in plant operation could be achieved in comparison to 40wt% NaGly. Just like with NaGly, the difficult feasibility of KGly regeneration leads to slow CO2 absorption rates, resulting in a huge energy demand >5.5 GJ/tCO2. A far lower energy demand of 3.68 GJ/tCO2 was measured for 30wt% MEA. The required solvent flow rate for reaching the optimal operating point is high (L/G-ratio=9l/m3). The pilot plant was operated with a massive reduced flue gas flow (40% of the dimensioning) in order to prevent instable operation and to reach the desired CO2 separation efficiency of 90%. During a prolonged shutdown, excessive carbonate was formed in the desorber column, resulting in a further reduction of the flue gas flow rate. Measurements of the ammonia content of the treated flue gas exhibit that the solvent degrades enormously.

Naproxen–Nicotinamide Cocrystals: Racemic and Conglomerate Structures Generated by CO2 Antisolvent Crystallization

Cocrystallization of naproxen racemic mixture and nicotinamide was investigated in this work, using compressed CO2 as antisolvent. A novel racemic cocrystal structure containing both enantiomers of naproxen linked to nicotinamide has been produced thanks to the CO2 antisolvent batch crystallization process. The structure of the molecular complex and its intermolecular interactions were investigated by single-crystal X-ray diffraction and attenuated total reflectance Fourier transform infrared spectroscopy. The antisolvent feed rate was found to have a direct influence on the cocrystallization outcome. The racemic cocrystal was obtained at slow and moderate CO2 feed rate, while very fast introduction of CO2 resulted in the formation of a mixture of chiral cocrystals (conglomerate). Cross-seedings, thermal analysis, and temperature-resolved X-ray powder diffraction were used to probe the relationship between the different phases. In addition, all powders produced with CO2 technology were obtained as cocryst...

Reducing uncertainty associated with CO2 injection and brine production in heterogeneous formations

Spatial heterogeneity and variability across many orders of magnitude are two properties of formation permeability that are challenging to effectively capture in subsurface flow models. With regard to geological CO2 storage, heterogeneity affects storage capacity, plume migration, pressure build-up around injection wells and, if utilized, the possibility of early breakthrough at brine extraction wells. Faced with under-characterized heterogeneity and an information-poor environment, the challenge is to make as much use as possible of the available data in reducing uncertainty associated with model prediction of a CO2 injection operation. The two types of data used here to inform models are point permeability measurements from each well and interference tests between wells; these are both obtained from a synthetic permeability distribution approximating a hypothetical CO2 storage formation. These data are used to reduce uncertainty in predictions of CO2 injection, brine and CO2 production rates, and CO2 breakthrough times obtained from simulations of five-spot CO2 injection/brine production performed on >600 conditional permeability realizations. First, assuming the underlying permeability field exhibits long-range correlation and a log-normal permeability distribution, a Bayesian analysis was undertaken using the point permeability measurements to reduce the likely magnitude of permeability heterogeneity. Comparison of realizations drawn from the prior and posterior indicate that the data were sufficient to exclude extreme scenarios with very rapid or very slow CO2 transport and breakthrough at brine production wells. Thirty-year prediction envelopes for net CO2 retention, brine and CO2 production were reduced by 40, 45 and 72%, respectively. In the second stage, simulated interference tests for each of the posterior realizations were compared against interference tests simulated for the synthetic “true” permeability field. Weighting functions derived from these comparisons are used to further refine predictions, providing additional reductions of uncertainty by 38 and 69% for brine and CO2 production rates, respectively. Predictions of CO2 breakthrough are especially sensitive to analysis of interference tests, with a good estimate of the correct time obtained in three out of four cases.

Thermodynamic properties of CO2 during controlled decompression of supercritical extraction vessels

Very fast decompressions of extraction vessels reduce the temperature of their walls below the −10°C limit when they become brittle. Therefore, decompression of large vessels operating at high pressure may set a lower limit to extraction time in industrial multi-vessel supercritical CO2 extraction plants (thus negatively influencing production cost), but this has rarely been studied. This work studied decompression of CO2 initially at 30MPa in a 3.96L vessel vented through a partially opened needle valve. Slow CO2 venting avoided a two-phase (liquid–gas) mixture in the vessel. Pressure drop was fast at the beginning and end of decompressions, and comparatively slower within the 10–15MPa interval, before CO2 crossed the pseudocritical line. Temperature dropped at the beginning of decompressions, remained comparatively constant in the intermediate period, and increased at the end. Decompression time was inversely proportional to valve opening, and was shorter when starting at 60 than 50°C. The initial drop in temperature was proportional to valve opening, and similar for different starting temperatures. When the vessel was packed with pretreated (either pressed or pressed and pelletized) and extracted rosehip or raspberry seeds, depressurization time was proportional to the porosity of the packed bed and independent on other properties of the substrate. Being flow through the needle valve choked, venting rate was determined by its opening and the continuously changing physical properties of CO2 in the vessel. To estimate the changes in internal energy of the CO2 in the vessel that define vessel wall's temperature, the heat provided by the service fluid (in our case water circulated through the vessel jacket), the heat yield by the walls, lid, and bottom of the high-pressure vessel, and the heat yield by the substrate charge need to be incorporated to the transient heat balance model of the depressurization process. The experimental results obtained in this work can be used to validate a model of the depressurization process that could simulate decompressions of industrial vessels as a function of their size and shape, the extraction conditions, and the packed substrate. Predicted decompression times would therefore set restrictions to the setup times required to replace exhausted by fresh substrate in industrial vessels, in studies of minimization of production cost.

The optimally performing Fischer-Tropsch catalyst.

Microkinetics simulations are presented based on DFT-determined elementary reaction steps of the Fischer-Tropsch (FT) reaction. The formation of long-chain hydrocarbons occurs on stepped Ru surfaces with CH as the inserting monomer, whereas planar Ru only produces methane because of slow CO activation. By varying the metal-carbon and metal-oxygen interaction energy, three reactivity regimes are identified with rates being controlled by CO dissociation, chain-growth termination, or water removal. Predicted surface coverages are dominated by CO, C, or O, respectively. Optimum FT performance occurs at the interphase of the regimes of limited CO dissociation and chain-growth termination. Current FT catalysts are suboptimal, as they are limited by CO activation and/or O removal.

Structure sensitivity in the ruthenium nanoparticle catalyzed aqueous-phase Fischer–Tropsch reaction

Low-temperature Fischer–Tropsch reaction data are reported for Ru nanoparticles suspended in the water phase. Their activity and selectivity strongly depends on particle size, when varied between 1 to 5 nm. Small particles display high oxygenates selectivity. The Anderson–Schulz–Flory (ASF) chain-growth probability for oxygenates is significantly lower than that observed for hydrocarbons. The chain growth parameter for hydrocarbon formation is independent of particle size. For oxygenates it is constant only for particles larger than 3 nm. Oxygenate and hydrocarbon formation occur on different sites. The ASF chain-growth probability for oxygenate formation increases with temperature. For very small 1.2 nm particles it shows a maximum as a function of temperature. This unusual temperature dependence is due to relatively slow CO dissociation compared to the rate of C–C bond formation.

LES investigation of swirl intensity effect on unconfined turbulent swirling premixed flame

A finite reaction rate model is presented as a closure of large eddy simulation (LES) to numerically study an open premixed methane/air swirling flame. The resultant model is firstly validated by comparing with reported data and then employed to investigate the effect of swirling intensity on flow field, flame characteristics and combustion instability of the swirling flame. Three different swirl numbers are considered. The LES results show that as swirling intensity increases, the vortex entrainment and micro-mixing are enhanced, leading to more lean equivalent ratios at flame front; consequently, higher swirling number causes lower flame temperatures and slower CO oxidization; for all simulated swirl numbers, flame fronts are completely located out of the recirculation zones and anchored at the inner surface of the annular swirling steams; swirl number has a crucial effect on swirling flame extension toward radial and tangential dimensions and then significantly affects streamwise flame length, which is a great influencing factor on combustion instability; vortex-induced disturbance on flame in streamwise plays a critical role in combustion instability.

H2 production by sorption enhanced steam reforming of biomass-derived bio-oil in a fluidized bed reactor: An assessment of the effect of operation variables using response surface methodology

High-purity H2 was produced by the sorption enhanced steam reforming (SESR) of acetic acid, a model compound of bio-oil obtained from the fast pyrolysis of biomass, in a fluidized bed reactor. A Pd/Ni–Co hydrotalcite-like material (HT) and dolomite were used as reforming catalyst and CO2 sorbent, respectively. The hydrogen yield and purity were optimized by response surface methodology (RSM) and the combined effect of the reaction temperature (T), steam-to-carbon molar ratio in the feed (steam/C) and weight hourly space velocity (WHSV) upon the sorption enhanced steam reforming process was analyzed. T was studied between 475 and 675°C, steam/C ratio between 1.5 and a 4.5mol/mol and WHSV between 0.893 and 2.679h−1. H2 yield, H2 selectivity and H2 purity, as well as the CH4, CO and CO2 concentrations in the effluent gas, were assessed. The operating temperature proved to be the variable that had the greatest effect on the response variables studied, followed by the WHSV and the steam/C ratio. The results show that the H2 yield, H2 selectivity and H2 purity increased, while the CH4, CO and CO2 concentrations decreased, concurrently with the temperature up to around 575–625°C. Higher values of the steam/C ratio and lower WHSV values favored the H2 yield, H2 selectivity and H2 purity, and reduced the CH4 concentration. It was found that the SESR of acetic acid at atmospheric pressure and 560°C, with a steam/C ratio of 4.50 and a WHSV of 0.893h–1 gave the highest H2 yield of 92.00%, with H2 purity of 99.53% and H2 selectivity of 99.92%, while the CH4, CO and CO2 concentrations remained low throughout (0.04%, 0.06% and 0.4%, respectively). The results also suggested that a slow CO2 capture rate led to a poor level of hydrogen production when the SESR process was carried out at low temperatures, although this can be improved by increasing the sorbent/catalyst ratio in the fluidized bed.

State-resolved imaging of CO from propenal photodissociation: Signatures of concerted three-body dissociation

State-selected DC sliced images of propenal photodissociation show clear signatures of a novel synchronous concerted three-body dissociation of propenal recently proposed by Lee and co-workers to give C2H2 + H2 + CO [S. H. Lee, C. H. Chin, C. Chaudhuri, ChemPhysChem 12, 753 (2011)]. Unlike any prior example of a concerted 3-body dissociation event, this mechanism involves breaking three distinct bonds and yields 3 distinct molecules. DC sliced images of CO fragments were recorded for a range of rotational levels for both v = 0 and v = 1. The results show formation of two distinct CO product channels having dissimilar translational energy distributions with characteristic rovibrational state distributions. The images for CO (v = 0) show a large contribution of slower CO fragments at lower rotational levels (J = 5–25). This slow component is completely absent from the v = 1 CO images. The images for the higher rotational levels of the v = 0 and v = 1 CO are nearly identical, and this provides a basis for decomposing the two channels for v = 0. The quantum state and translational energy distributions for the slow channel are readily assigned to the 3-body dissociation based on the properties of the transition state. The faster CO fragments dominating the higher rotational levels in both v = 0 and v = 1 are attributed to formation of CH3CH + CO, also in agreement with the inferences based on previous non-state-resolved measurements with supporting theoretical calculations.

Experimental and Modeling Study of the Thermal Decomposition of C3–C5 Ethyl Esters Behind Reflected Shock Waves

The thermal decomposition of three ethyl esters, ethyl formate (C3H6O2), ethyl acetate (C4H8O2), and ethyl propanoate (C5H10O2), was studied behind reflected shock waves using laser absorption to measure concentration time-histories of H2O, CO2, and CO. Experimental conditions covered temperatures of 1301-1636 K, pressures of 1.48-1.72 atm, and reactant concentrations of 2000 ppm in argon. Recently developed mid-infrared laser diagnostics for H2O (2.5 μm), CO2 (4.3 μm), and CO (4.6 μm) provide orders-of-magnitude greater detectivity compared to previous near-infrared absorption sensors. The experimental results have highlighted significant differences among these three ethyl esters: negligible CO2 production during ethyl formate pyrolysis, quite slow CO formation rate during ethyl acetate pyrolysis, and nearly equal formation rate of H2O, CO2, and CO during ethyl propanoate pyrolysis. Detailed kinetic modeling was performed to understand the destruction pathways of these three ethyl esters with different alkyl chain lengths. Rate of production and sensitivity analyses were also carried out to interpret the experimental results and to identify the key reactions affecting experimental results.

Modelling choked flow for CO2 from the dense phase to below the triple point

The modelling of the choked flow for CO2 undergoing expansion from the dense phase state to below its triple point is presented. The choked flow properties, including mass flux, fluid phase composition, pressure, speed of sound and solids mass fraction are determined for the range of stagnation conditions relevant to the pipeline transportation of CO2 following its accidental failure. The data indicate that the decompression of the dense phase CO2 results in the stagnation of the flow at its triple point. At pressures below the triple point, the solid/vapour mixture is shown to be released at a mass flux of ca. 2000kg/(m2s). For a 50mm diameter puncture in a pipeline transporting CO2 at its critical point (304.2K, 73.83bar), the above translates into a discharge rate of ca. 1kg/s of solid CO2. Under such circumstances, in the case of buried pipelines, the accumulation of the slow subliming solid CO2 within the resulting crater may significantly modify the subsequent dispersion behaviour of the migrating CO2 cloud. As such, the phase transition behaviour of CO2 around its triple point must be accounted for when performing safety assessment of CO2 pipelines being considered as part of the carbon capture and sequestration chain.